Circuit interrupters may be used as disconnecting switches for positively disconnecting an electrical power transmission line from a source of power. Disconnection is often required to enable maintenance work to be performed on the transmission line or on an electrical apparatus connected to the transmission line. Interrupters typically have two contacts that are in physical contact with one another when the transmission line is connected to the power source and are not in physical contact when the transmission line is disconnected from the power source. The interrupter is said to be in the closed circuit position when the contacts are in contact and in the open circuit position when the contacts are not in contact. Because one of the interrupter's contacts typically fits into the other contact in the closed circuit condition, the contacts are usually referred to as "penetrating contacts," including separable male and female contacts. Specifically, a conventional set of penetrating contacts includes a pin contact (male) and a tulip contact (female).
Typically, electric power transmission and distribution lines that carry high voltage and/or high current must be disconnected quickly in order to avoid a restrike. Restrikes occur when the interrupter's contacts are not connected, but are still close enough to each other to permit current to be conducted through the air (or other media) between the contacts. When the contacts of a properly designed penetrating contact are fully separated, the distance between the contacts is sufficient to prohibit a restrike. However, a restrike can occur as the contacts are moved from the fully connected position to the fully separated position. Likewise, a restrike can occur as the contacts are moved from the fully separated position to the fully connected position.
Circuit interrupt designers seek to minimize restrikes because restrikes can be dangerous to persons operating the interrupter, can cause system disturbances, and can degrade the components of the interrupter. Thus, the separation mechanism of an interrupter must be capable of opening the contacts at a separation velocity sufficient to prevent a restrike of the arc once the initial arc extinction occurs at a current zero. Separation acceleration is typically provided by separation mechanisms such as a spring arrangement in the circuit interrupter. The potential energy stored in a spring-type separation mechanism is used to produce the kinetic energy necessary to provide the desired separation velocity.
Once the circuit is opened, there is a rapid rise in voltage across the contacts known as the "transient recovery voltage." If the contacts are not separated quickly enough for the gap between the contacts (the "arc gap") to withstand this rising voltage, then the gap breaks down and the resulting current flow results in a restrike. Restrikes generally occur at or near the point when the transient recovery voltage reaches its maximum value. Thus, to prevent a restrike, the contacts must be moved from the fully connected position to a position at which a restrike is impossible within a period that is less than the time it takes the voltage to reach its maximum value.
Thus, it is a goal of interrupt designers to design an interrupt with a separation mechanism that can store sufficient potential energy prior to contact separation to provide a separation acceleration sufficient to prevent a restrike. However, cost is another design constraint and feasible separation mechanisms must be limited to those that will provide an acceptable separation acceleration, but are cost effective.
Conventional penetrating contacts used in interrupters include models providing a high degree of resistance to separation (e.g., static friction) between the male and female contacts when the contacts are in a closed circuit position. Penetrating contacts of this design permit the separation mechanism to store additional potential energy prior to separating the contacts, because the contacts are held in the closed circuit position by the high degree of separation resistance. The additional potential energy helps to generate a separation acceleration suitable for minimizing restrikes. Unfortunately, available contacts of this kind generate a dynamic drag as the contacts accelerate toward separation which effectively reduces the separation acceleration, thus increasing the chances of generating a restrike following separation.
Another kind of penetrating contact arrangement does not create a high degree of separation resistance. However, with contacts of this type, the separation mechanism must generate a relatively high energy level to produce an acceptable separation acceleration. Such separation mechanisms tend to have complex designs and are expensive to manufacture and maintain.
Non-penetrating contacts could also be used in circuit interrupters with spring-type separation mechanisms in order to reduce dynamic drag. However, non-penetrating interrupters do not provide sufficient resistance suitable for increasing the potential energy stored in the separation mechanism. Thus, the separation mechanism must produce the separation acceleration without the assistance of the separation resistance between the contacts. Such separation mechanisms are typically more expensive than those used with penetrating contacts. Often the increased cost of this kind of separation mechanism renders the interrupter design prohibitively expensive.
Sacrificial "butt" contacts have also been used in circuit interrupters as non-penetrating contacts. Sacrificial contacts are designed to deteriorate over time. Thus, there is no need for high speed contact separation, when sacrificial contacts are used in an interrupter. While this reduces the cost of the separation mechanism, the deterioration of the contacts reduces the conduction characteristics of the contacts and requires regular maintenance to monitor and replace the contacts as they deteriorate.
Therefore, there is a need for an interrupter that provides a high separation acceleration to minimize restrikes and contact deterioration. More particularly, there is a need for electrical contacts that can be implemented in an interrupter to provide for increased contact separation acceleration without increasing the required force of the separation mechanism. The contacts should have a simple structure and be inexpensive to manufacture.